Birefringence characteristic measuring method, optical recording medium and optical information recording/reproducing apparatus

ABSTRACT

Four types of optical systems, a polarization optical system including an objective lens having a high numerical aperture, a polarization optical system including an objective lens having a low numerical aperture, a non-polarization optical system including an objective lens having a high numerical aperture and a non-polarization optical system including an objective lens having a low numerical aperture, are selectively used at the time of irradiating light from a semiconductor laser on a target disk for measurement, and the in-plane birefringence characteristic and perpendicular birefringence characteristic of the target disk are separately acquired based on the amounts of received light obtained by measuring reflected light from the target disk by a photosensor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a birefringence characteristicmeasuring method, an optical recording medium and an optical informationrecording/reproducing apparatus, and, more particularly, to abirefringence characteristic measuring method which can separatelymeasure the in-plane birefringence and perpendicular birefringence inthe birefringence characteristic of the protective layer of an opticalrecording medium, an optical recording medium which has a protectivelayer with an excellent birefringence characteristic and has anexcellent recording/reproduction characteristic and an opticalinformation recording/reproducing apparatus which uses the opticalrecording medium.

[0003] 2. Description of the Related Art

[0004] According to the specifications for DVDs (Digital VersatileDisks) produced in 1996, the wavelength of a light source is 650 nm, thenumerical aperture (NA) of an objective lens is 0.6, the thickness of asubstrate which is the protective layer of an optical recording mediumis 0.6 mm and the recording capacity of the optical recording mediumwith a diameter of 120 mm is 4.7 GBytes. As the bit length is 0.267 μmand the track pitch is 0.74 μm, the recording density is 1/(0.267×0.74)bits/μm²=3.3 Gbits/inch².

[0005] However, the present recording density of DVDs is insufficient torecord and play back high-definition moving pictures for a long periodof time. Recording and playback of high-definition moving pictures at ahigh quality requires a data transfer rate of at least 13 Mbits/sec. Therecording density that is needed to record and play back moving picturesfor 120 minutes at this data transfer rate is 13 Mbits/sec×120minutes=11.7 GBytes. With the DVD standard, the recording density atthis time is calculated to be 3.3 Gbits/inch²×(11.7 GBytes/4.7GBytes)=8.2 Gbits/inch².

[0006] To increase the recording capacity of an optical recordingmedium, it is effective to shorten the wavelength of a light source tobe used in recording and playback. The recording capacity is inverselyproportional to the square of the diameter of a focused spot formed onan optical recording medium and the diameter of the focused spot isproportional to the wavelength of the light source. That is, therecording capacity is inversely proportional to the square of thewavelength of the light source. Recently, semiconductor lasers with awavelength of 405 nm or so, as described in Japanese Journal of AppliedPhysics, Vol. 39, Part 2, No.7A, pp. L647 to L650, and lasers which usessecond harmonics with a wavelength of 410 nm or so, as described inInternational Symposium on Optical Memory 2001 Technical Digest, pp. 228to 229, have achieved the practical levels.

[0007] With the DVD standard, the wavelength of a light source forachieving the recording capacity of 11.7 GBytes is 650 nm×{squareroot}{square root over ( )}(4.7 GBytes/11.7 GBytes)=412 nm. Therefore,the use of the aforementioned semiconductor laser and the aforementionedlaser using the second harmonics as a light source can record and playback high-definition moving pictures on and from an optical recordingmedium with a diameter of 120 mm, the same diameter as that of a DVD,for 120 minutes. As the numerical aperture (NA) of the objective lenscan be 0.6 and the thickness of the protective layer of the opticalrecording medium can be 0.6 mm, both being the same as those of a DVD,the objective lens and optical recording medium can be fabricated by thesame technology as used for DVDs. If the wavelength of the light sourceis made shorter than 412 nm and the numerical aperture (NA) of theobjective lens is made higher than 0.6, the diameter of the focused spotbecomes smaller, thus ensuring a recording capacity greater than 11.7GBytes (higher recording density than 8.2 Gbits/inch²).

[0008] While low-cost polycarbonate is generally used for a substratewhich is the protective layer of an optical recording medium, thepolycarbonate has birefringence. In an optical informationrecording/reproducing apparatus which records and plays back informationon and from an optical recording medium, a polarization optical systemwhich uses a combination of a polarization beam splitter and aquarter-wave plate is generally used to improve the utilization factorof light. In case where such a polarization optical system is used, ifthe protective layer of an optical recording medium has birefringence,the amount of received light of the photosensor which receives reflectedlight from the optical recording medium is reduced. Further, the peakintensity of the focused spot to be formed on the optical recordingmedium drops. The reduction in the amount of received light leads to adecrease in signal-to-noise ratio at the time of playback and thereduction in peak intensity leads to an increase in optical powerneeded.

[0009] According to the DVD specifications, therefore, a birefringencecharacteristic measuring method and the allowance for the birefringenceare defined for the protective layer of an optical recording medium.FIG. 1 shows the structure of an optical system which is used in thismeasuring method. The structure of the optical system is described inthe specifications for read-only DVD-ROM, “DVD Specifications forRead-Only Disc Part 1: PHYSICAL SPECIFICATIONS”, the specifications forrecordable DVD-R, “DVD Specifications for Recordable Disc Part 1:PHYSICAL SPECIFICATIONS”, the specifications for re-recordable DVD-RW,“DVD Specifications for Re-recordable Disc Part 1: PHYSICALSPECIFICATIONS” and so forth.

[0010] The light that is output from a laser 24 with a wavelength of 650nm as the light source is linearly polarized by a polarizer 25, isconverted to be circularly polarized by a quarter-wave plate 26 and isthen irradiated on a disk 6 as an optical recording medium. The angle ofincidence to the disk 6 is 7°. The reflected light from the disk 6 isreceived at a photosensor 29 via a rotation analyzer 27 and a collimatorlens 28. As the reflected light from the disk 6 reciprocates theprotective layer of the disk 6, it is influenced by the birefringenceand is elliptically polarized. By rotating the rotation analyzer 27 tomeasure the amount of light received at the photosensor 29, theellipticity of the elliptically polarized light is measured and abirefringence-originated phase difference δ between two orthogonalpolarized light components is acquired. Given that Δn is thebirefringence, d is the thickness of the protective layer and λ is thewavelength of the light source, as δ=(2π/λ)·Δn·2d is satisfied, thebirefringence Δn can be acquired from the equation. The specificationsof DVD-ROM, DVD-R and DVD-RW describe the allowance for birefringence asΔn·2d≦100 nm. As d=0.6 mm, Δn≦8.3×10⁻⁵.

[0011] As described in “Optics”, Vol. 15, No. 5, pp. 414 to 421, thebirefringence of the protective layer of an optical recording mediumincludes in-plane birefringence and perpendicular birefringence. Therelationship between the disk 6 as an optical recording medium and theXYZ coordinates is defined as shown in FIG. 2. The X axis, Y axis and Zaxis are respectively the radial direction, the tangential direction andthe normal direction of the disk 6. The protective layer of the opticalrecording medium normally has biaxial anisotropy and its three principalaxes approximately match with the X axis, Y axis and Z axis. Given thattheir associated three principal indexes of refraction are nx, ny andnz, respectively, and the in-plane birefringence and perpendicularbirefringence are Δn∥ and Δn⊥, respectively, the in-plane birefringenceis defined as Δn∥=|nx−ny| and the perpendicular birefringence asΔn⊥=|(nx+ny)/2−nz|.

[0012] The in-plane birefringence and perpendicular birefringence bothreduce the amount of received light at the photosensor which receivesreflected light from the optical recording medium and lower the peakintensity of the focused spot to be formed on the optical recordingmedium. However, the degree of influence on the light which passesthrough the protective layer of the optical recording medium differsbetween the in-plane birefringence and perpendicular birefringence.While the influence of the in-plane birefringence does not depend on theincident angle, the influence of the perpendicular birefringence does,and the light with an incident angle of 0° is not influenced but theinfluence gets greater as the incident angle increases.

[0013] In the method of measuring the birefringence characteristic ofthe protective layer of the conventional optical recording mediumdescribed above referring to FIG. 1, the incident angle of to the disk 6is as small as 7° so that the reflected light from the disk 6 isinfluenced by the in-plane birefringence but is hardly influenced by theperpendicular birefringence. Therefore, the in-plane birefringence isthe only birefringence that is measured by this measuring method. Theallowance for birefringence that is determined based on this measuringmethod is the allowance for the in-plane birefringence and the allowancefor the perpendicular birefringence is not determined. In case whererecording and playback of an optical recording medium whose protectivelayer has a thickness of 0.6 mm are carried out using the opticalinformation recording/reproducing apparatus whose light source has awavelength of 412 nm and whose objective lens has a numerical aperture(NA) of 0.6, if either one of the in-plane birefringence and theperpendicular birefringence of the protective layer of the opticalrecording medium is greater than the allowance, the amount of receivedlight and the peak intensity decrease, so that the recording density of8.2 Gbits/inch² (the recording capacity of 11.7 GBytes) cannot beachieved.

[0014] To suppress reduction in the amount of received light and thepeak intensity and achieve the recording density of 8.2 Gbits/inch²while using the optical information recording/reproducing apparatuswhose light source has a wavelength of 412 nm and whose objective lenshas a numerical aperture (NA) of 0.6, it is necessary to separatelymeasure the in-plane birefringence and perpendicular birefringence inthe birefringence characteristic of the protective layer of the opticalrecording medium, and set the allowance for the birefringencecharacteristic separately for the in-plane birefringence andperpendicular birefringence. In addition, the optical recording mediumshould have a good protective layer whose birefringence characteristicsatisfies the allowances for both the in-plane birefringence and theperpendicular birefringence and which has an excellentrecording/playback characteristic.

[0015] As a birefringence characteristic measuring method, the incidentangle to the disk 6 in the conventional measuring method may be setgreater than 7°. If the incident angle to the disk 6 is increased,however, a large phase difference arises on the reflection film betweentwo orthogonal polarized light components, making it impossible todistinguish the birefringence-originated phase difference from the phasedifference on the reflection film. Apparently, this method cannotmeasure the birefringence accurately. Another feasible measuring methodis to use a protective layer before deposition of a reflection filminstead of the disk 6 in the conventional measuring method, set theincident angle to the protective layer greater than 7° and measure theellipticity of the elliptically polarized light transmitted through theprotective layer instead of the ellipticity of the ellipticallypolarized light reflected at the disk 6. As the deposition of thereflection film causes the birefringence of the protective layer to varydepending on the stress or the like of the reflection film, however,this method does not accurately measure the birefringence of theprotective layer of the disk 6.

SUMMARY OF THE INVENTION

[0016] Accordingly, it is an object of the invention to overcome theproblems of the conventional methods of measuring the birefringencecharacteristic of the protective layer of an optical recording mediumand provide a birefringence characteristic measuring method which canseparately measure the in-plane birefringence and perpendicularbirefringence in the birefringence characteristic of the protectivelayer of an optical recording medium, an optical recording medium whichhas a protective layer with an excellent birefringence characteristicand has an excellent recording/playback characteristic and an opticalinformation recording/reproducing apparatus using the optical recordingmedium in order to record and play back high-definition moving picturesfor 120 minutes.

[0017] To achieve the object, a birefringence characteristic measuringmethod according to the first aspect of the invention comprises thesteps of irradiating light onto a target medium for measurement via anobjective lens having a numerical aperture equal to or higher than apredetermined numerical aperture, and measuring an amount of light of apolarized light component in a specific direction, which is included inreflected light reflected at a reflection surface of the target mediumto thereby acquire a first amount of light A_(PH); irradiating lightonto the target medium via the objective lens having the numericalaperture equal to or higher than the predetermined numerical aperture,and measuring an amount of light of a polarized light component in thespecific direction, which is included in reflected light reflected atthe reflection surface of the target medium, and an amount of light of apolarized light component in a direction orthogonal to the specificdirection to thereby acquire a second amount of light A_(NH); andacquiring a perpendicular birefringence characteristic of the targetmedium based on a ratio A_(PH)/A_(NH) of the first amount of light tothe second amount of light and an in-plane birefringence characteristicof the target medium.

[0018] According to the birefringence characteristic measuring method ofthe first aspect of the invention, the first amount of light and thesecond amount of light can be measured by guiding reflected light fromthe target medium to the photosensor via, for example, a polarizationoptical system and non-polarization optical system. Based on the ratioof the first amount of light to the second amount of light, therefore,the perpendicular birefringence characteristic of the target medium canbe acquired.

[0019] A value selected for the predetermined numerical aperture is suchthat with the numerical aperture below that value, a reduction in theamount of received light influenced by the perpendicular birefringencein the polarization optical system is substantially negligible and whenthe numerical aperture is equal to or higher than the value, thereduction in the amount of received light influenced by theperpendicular birefringence rapidly increases with an increase innumerical aperture. For example, 0.4 is selected for the predeterminednumerical aperture. A numerical aperture equal to or higher than thepredetermined numerical aperture is, for example, 0.5 or higher, andpreferably is 0.6 or higher.

[0020] In the measuring method according to a preferable mode, light isirradiated onto the target medium via an objective lens having anumerical aperture lower than the predetermined numerical aperture, andreflected light reflected at the reflection surface of the target mediumis irradiated on a photosensor via a polarization optical system, andthe amount of light of a polarized light component in a specificdirection, which is included in reflected light is measured to therebyacquire a third amount of light A_(PL),

[0021] light is irradiated onto the target medium via the objective lenshaving the numerical aperture lower than the predetermined numericalaperture, reflected light reflected at the reflection surface of thetarget medium is irradiated on the photosensor via a non-polarizationoptical system, and the amount of light of a polarized light componentin the specific direction, which is included in the reflected light andthe amount of light of a polarized light component in a directionorthogonal to the specific direction are measured to thereby acquire afourth amount of light A_(NL), and

[0022] the in-plane birefringence characteristic is acquired based on aratio A_(PL)/A_(NL) of the third amount of light to the fourth amount oflight. As the in-plane birefringence can be measured by merely changingthe objective lens of each optical system, the birefringencecharacteristic is obtained easily. For example, 0.3 is selected for anumerical aperture (NA) lower than the predetermined numerical aperture.

[0023] According to another preferable mode of the invention, theoptical system that is used at the time of measuring the first amount oflight is constructed by a polarization optical system which outputs apolarized light component in the specific direction in the incidentreflected light, and

[0024] the optical system that is used at the time of measuring thesecond amount of light is constructed by a non-polarization opticalsystem which outputs a polarized light component in the specificdirection in the incident reflected light and a polarized lightcomponent in a direction orthogonal to the specific direction at anapproximately same ratio. For example, the ratio can be set to 50%.

[0025] According to a further preferable mode of the invention, first tofourth amounts of light for normalization B_(PH), B_(NH), B_(PL) andB_(NL) respectively corresponding to the first to fourth amounts oflight are measured using a reference medium which does not substantiallyhave birefringence in place of the target medium, and the first tofourth amounts of light A_(PH), A_(NH), A_(PL) and A_(NH) arerespectively normalized based on the first to fourth amounts of lightfor normalization. Because the utilization factor of light differsbetween a polarization optical system and a non-polarization opticalsystem, the execution of such normalization can provide a more accuratebirefringence characteristic.

[0026] A birefringence characteristic measuring method according to thesecond aspect of the invention measures a birefringence characteristicof a protective layer of an optical recording medium using abirefringence characteristic measuring apparatus having a light source,a photosensor, and an optical system which includes a beam splitter, aquarter-wave plate for passing light output from the light source andpassed through the beam splitter and an objective lens for irradiatingthe light, transmitted through the quarter-wave plate, onto a targetoptical recording medium for measurement and which guides reflectedlight reflected from the target optical recording medium to thephotosensor via the objective lens, the quarter-wave plate and the beamsplitter, and is characterized in that the optical system is switchedamong

[0027] a first optical system which guides only a polarized lightcomponent in a specific direction in the reflected light reflected fromthe target optical recording medium using an objective lens having anumerical aperture equal to or higher than a predetermined numericalaperture,

[0028] a second optical system which guides only a polarized lightcomponent in the specific direction in the reflected light reflectedfrom the target optical recording medium using an objective lens havinga numerical aperture lower than the predetermined numerical aperture,

[0029] a third optical system which guides a polarized light componentin the specific direction and a polarized light component in a directionorthogonal to the specific direction, both included in the reflectedlight reflected from the target optical recording medium, at anapproximately same ratio, using an objective lens having a numericalaperture equal to or higher than the predetermined numerical aperture,and

[0030] a fourth optical system which guides a polarized light componentin the specific direction and a polarized light component in a directionorthogonal to the specific direction, both included in the reflectedlight reflected from the target optical recording medium, at anapproximately same ratio, using an objective lens having a numericalaperture lower than the predetermined numerical aperture.

[0031] The in-plane birefringence characteristic and perpendicularbirefringence characteristic both can be acquired effectively bymeasuring the amount of light of a polarized light component in aspecific direction and the amounts of lights of polarized lightcomponents in a specific direction and a direction orthogonal to thespecific direction using the first to fourth optical systems. The firstto fourth optical systems may be constructed separately or with commonparts used, only a specific part may be changed as needed.

[0032] In the birefringence characteristic measuring method according tothe second aspect of the invention, it is preferable that based onamounts of received light measured for a reference optical recordingmedium having a protective layer which does not substantially havebirefringence, using the first to fourth optical systems, amounts ofreceived light measured for the target optical recording medium usingthe first to fourth optical systems should be normalized respectively.

[0033] Given that the amounts of received light measured for the targetoptical recording medium by the first to fourth optical systems arerespectively A_(PH), A_(PL), A_(NH) and A_(NL) and the amounts ofreceived light measured for the reference optical recording medium bythe first to fourth optical systems are respectively B_(PH), B_(PL),B_(NH) and B_(NL), an in-plane birefringence characteristic and aperpendicular birefringence characteristic of the target opticalrecording medium can be acquired respectively from

L∥=(A _(PL) /B _(PL))/(A _(NL) /B _(NL)) , and

L⊥=[(A _(PH) /B _(pH))/(A _(NH) /B _(NH))]/[(A _(PL) /B _(PL))/(A _(NL)/B _(NL))].

[0034] As L∥0 and L⊥ respectively represent the relative amount ofreceived light including the influence of only the in-planebirefringence and the relative amount of received light including theinfluence of only the perpendicular birefringence, with the amount ofreceived light in case of no birefringence present taken as a reference,the in-plane birefringence characteristic and the perpendicularbirefringence characteristic can be acquired separately.

[0035] In the birefringence characteristic measuring method according tothe second aspect of the invention, if the protective layer of theoptical recording medium has birefringence, the amounts of receivedlights in the first and second optical systems decrease while theamounts of received lights in the third and fourth optical systems donot. Therefore, the relative amount of received light in case wherethere is birefringence, with the amount of received light in case of nobirefringence present taken as a reference, can be acquired from theratio of the amounts of received lights in the first and second opticalsystems to the amounts of received lights in the third and fourthoptical systems. In each optical system, if the amount of received lightwith respect to a target optical recording medium for measurement isnormalized by the amount of received light with respect to the referenceoptical recording medium, a difference in the utilization factor oflight from the optical recording medium to the photosensor can becanceled out. In case of the second optical system whose objective lenshas a low numerical aperture, while the amount of received light isreduced by the influence of the in-plane birefringence, a reduction inthe amount of received light by the influence of the perpendicularbirefringence is negligible. In case of the first optical system whoseobjective lens has a high numerical aperture, a reduction in the amountof received light by the influence of the in-plane birefringence and areduction in the amount of received light by the influence of theperpendicular birefringence both occur. Accordingly, L∥ and L⊥respectively represent the relative amount of received light includingthe influence of only the in-plane birefringence and the relative amountof received light including the influence of only the perpendicularbirefringence, with the amount of received light in case of nobirefringence present taken as a reference. Therefore, the in-planebirefringence and perpendicular birefringence in the birefringencecharacteristic of the protective layer of the optical recording mediumcan be measured separately by measuring L∥ and L⊥.

[0036] An optical recording medium according to the first aspect of theinvention is an optical recording medium whose recording or playback iscarried out at a recording density of 8.2 Gbits/inch² or higher using anoptical information recording/reproducing apparatus having a lightsource with a wavelength of 412 nm or less and an objective lens with anumerical aperture of 0.6 or higher, and is characterized by having aprotective layer with a thickness of about 0.6 mm through which light istransmitted and whose L∥ obtained as L∥=(A_(PL)/B_(PL))/(A_(NL)/B_(NL))is L∥≧0.79.

[0037] An optical recording medium according to the second aspect of theinvention is an optical recording medium whose recording or playback iscarried out at a recording density of 8.2 Gbits/inch² or higher using anoptical information recording/reproducing apparatus having a lightsource with a wavelength of 412 nm or less and an objective lens with anumerical aperture of 0.6 or higher and is characterized by having aprotective layer with a thickness of about 0.6 mm through which light istransmitted and in which Δn∥·2d≦64 nm where Δn∥ is a value of in-planebirefringence of the protective layer and d is a thickness of theprotective layer.

[0038] An optical recording medium according to the third aspect of theinvention is an optical recording medium whose recording or playback iscarried out at a recording density of 8.2 Gbits/inch² or higher using anoptical information recording/reproducing apparatus having a lightsource with a wavelength of 412 nm or less and an objective lens with anumerical aperture of 0.6 or higher, and is characterized by having aprotective layer with a thickness of about 0.6 mm through which light istransmitted and whose L⊥ obtained asL⊥=[(A_(PH)/B_(PH))/(A_(NH)/B_(NH))]/[(A_(PL)/B_(PL))/(A_(NL)/B_(NL))]is L⊥≧0.57. Alternatively, this (A_(PL)/B_(PL))/(A_(NL)/B_(NL)) can beacquired by separately measuring the value of the in-plane birefringenceof the protective layer and converting the value to L∥.

[0039] An optical information recording/reproducing apparatus accordingto the first aspect of the invention has a light source with awavelength of 412 nm or less and an objective lens with a numericalaperture of 0.6 or higher and records or plays back, at a recordingdensity of 8.2 Gbits/inch² or higher, an optical recording medium whichhas a protective layer with a thickness of about 0.6 mm through whichlight is transmitted and whose L∥ acquired asL∥=(A_(PL)/B_(PL))/(A_(NL)/B_(NL)) is L∥≧0.79.

[0040] An optical information recording/reproducing apparatus accordingto the second aspect of the invention has a light source with awavelength of 412 nm or less and an objective lens with a numericalaperture of 0.6 or higher and records or plays back, at a recordingdensity of 8.2 Gbits/inch² or higher, an optical recording medium whichhas a protective layer with a thickness of about 0.6 mm through whichlight is transmitted and in which Δn∥·2d≦64 nm where Δn∥ is a value ofin-plane birefringence of the protective layer and d is the thickness ofthe protective layer.

[0041] An optical information recording/reproducing apparatus accordingto the third aspect of the invention has a light source with awavelength of 412 nm or less and an objective lens with a numericalaperture of 0.6 or higher and records or plays back, at a recordingdensity of 8.2 Gbits/inch² or higher, an optical recording medium whichhas a protective layer with a thickness of about 0.6 mm through whichlight is transmitted and whose L⊥ acquired asL⊥=[(A_(PH)/B_(PH))/(A_(NH)/B_(NH))]/[(A_(PL)/B_(PL))/(A_(NL)/B_(NL))]is L⊥≧0.57. Alternatively, this (A_(PL)/B_(PL))/(A_(NL)/B_(NL)) can beacquired by separately measuring the value of the in-plane birefringenceof the protective layer and converting the value to L∥.

[0042] In the optical recording medium according to the invention andthe optical information recording/reproducing apparatus using thatoptical recording medium, L∥≧0.79 and Δn∥·2d≦64 nm are equivalent to thecase where the amount of received light at the photosensor with thein-plane birefringence present is equal to or greater than the allowancefor the amount of light based on the allowance for birefringence ofDVDs. L⊥≧0.57 is equivalent to the case where the peak intensity of thefocused spot with the perpendicular birefringence present is equal to orgreater than the allowance for of the peak intensity based on theMarechal's criterion concerning wavefront aberration. When thewavelength of the light source is 412 nm or shorter, the numericalaperture of the objective lens is 0.6 or higher and the thickness of theprotective layer of the optical recording medium is 0.6 mm, therefore,it is possible to achieve the recording density of 8.2 Gbits/inch² orhigher which is equivalent to the square of the wavelength ratioaccording to the DVD specifications and is needed to record and playback high-definition moving pictures for 120 minutes.

[0043] In short, the invention can realize the birefringencecharacteristic measuring method capable of separately measuring thein-plane birefringence and perpendicular birefringence in thebirefringence characteristic of the protective layer of an opticalrecording medium, and an optical recording medium which has a protectivelayer with an excellent birefringence characteristic and has anexcellent recording/playback characteristic, and an optical informationrecording/reproducing apparatus using that optical recording medium inorder to record and play back high-definition moving pictures for 120minutes.

[0044] In other words, the birefringence characteristic measuring methodaccording to the invention can measure the perpendicular birefringenceof a target medium for measurement separately from the in-planebirefringence of the target medium, so that the method, if adapted tothe protective layer of an optical recording medium, for example, caneasily acquire the birefringence characteristic of the protective layerof the optical recording medium and can be utilized in manufacture anddesign of an optical recording medium and an optical informationrecording/reproducing apparatus.

[0045] According to the optical recording medium embodying the inventionand the optical information recording/reproducing apparatus which usesthis optical recording medium, the birefringence characteristic of theprotective layer of the optical recording medium satisfies theallowances for both the in-plane birefringence and perpendicularbirefringence and the recording density of 8.2 Gbits/inch² or higher isachieved, thus ensuring recording and playback of high-definition movingpictures for 120 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046]FIG. 1 is a diagram showing a conventional the birefringencecharacteristic of the protective layer of an optical recording medium;

[0047]FIG. 2 is a diagram showing the relationship between an opticalrecording medium and the XYZ coordinates;

[0048]FIG. 3 is a diagram illustrating the structure of a polarizationoptical system to be used in a method of measuring the birefringencecharacteristic of the protective layer of an optical recording mediumaccording to one embodiment of the invention;

[0049]FIG. 4 is a diagram illustrating the structure of anon-polarization optical system to be used in the method of measuringthe birefringence characteristic of the protective layer of the opticalrecording medium according to the embodiment of the invention;

[0050]FIG. 5 is a diagram illustrating the structure of a photosensor tobe used in the method of measuring the birefringence characteristic ofthe protective layer of the optical recording medium according to theembodiment of the invention;

[0051]FIG. 6 is a diagram showing an example of computation of therelationship between in-plane birefringence and the relative amount ofreceived light;

[0052]FIG. 7 is a diagram showing an example of computation of therelationship between in-plane birefringence and the relative peakintensity;

[0053]FIG. 8 is a diagram showing an example of computation of therelationship between perpendicular birefringence and the relative amountof received light;

[0054]FIG. 9 is a diagram showing an example of computation of therelationship between perpendicular birefringence and the relative peakintensity;

[0055]FIG. 10 is a diagram showing another example of computation of therelationship between perpendicular birefringence and the relative amountof received light; and

[0056]FIG. 11 is a diagram illustrating an optical informationrecording/reproducing apparatus according to one embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] The invention will be described below based on embodiments of theinvention referring to the accompanying drawings. A birefringencecharacteristic measuring method according to one embodiment of theinvention does not measure the birefringence of the protective layer of,for example, an optical recording medium itself, but irradiates outputlight from a light source onto an optical recording medium via anobjective lens and measures the amount of received light at aphotosensor when reflected light from the optical recording medium isreceived via the objective lens at the photosensor. As an optical systemincluding a light source, an objective lens and a photosensor, fourkinds are used, which are a polarization optical system whose objectivelens has a high numerical aperture (NA), a polarization optical systemwhose objective lens has a low numerical aperture (NA), anon-polarization optical system whose objective lens has a highnumerical aperture (NA) and a non-polarization optical system whoseobjective lens has a low numerical aperture (NA). Here, the“polarization optical system” is an optical system in which, of lightthat is the reflected light from an optical recording medium which haspassed through a quarter-wave plate, only a polarized light component ina specific direction is received by the photosensor, while the“non-polarization optical system” is an optical system in which, oflight that is the reflected light from an optical recording medium whichhas passed through a quarter-wave plate, a polarized light component ina specific direction and a polarized light component in a directionorthogonal to the specific direction are received by the photosensor atthe same ratio. As an optical recording medium, two types are used whichis a target disk for measurement the birefringence characteristic ofwhose protective layer is to be measured and a glass disk whoseprotective layer is glass that does not have birefringence. The glassdisk may be replaced with any other disk whose protective layer is madeof a material having no birefringence.

[0058] Table 1 below shows combinations of the four types of opticalsystems and the two types of optical recording media in the method ofmeasuring the birefringence characteristic of the protective layer of anoptical recording medium according to the embodiment of the invention.TABLE 1 Target disk Glass disk Polarization optical A_(PH) B_(PH) system(high NA) Polarization optical A_(PL) B_(PL) system (low NA)Non-polarization optical A_(NH) B_(NH) system (high NA) Non-polarizationoptical A_(NL) B_(NL) system (low NA)

[0059] As shown in Table 1, the amount of received light at thephotosensor is measured for each of the total of eight combinations. Theamounts of received lights at the photosensor for the individualcombinations are given by the corresponding symbols in the Table 1. L∥and L⊥ are acquired from those amounts of received lights by thefollowing equations.

L∥=(A _(PL) /B _(PL))/(A _(NL) /B _(NL))

L⊥=[(A _(PH) /B _(PH))/(A _(NH) /B _(NH))]/[(A _(PL) /B _(PL))/(A _(NL)/B _(NL))]

[0060] Without birefringence in the protective layer of the opticalrecording medium, reflected light from the optical recording mediumwhich is received at the photosensor has only a polarized lightcomponent in the specific direction. With birefringence in theprotective layer of the optical recording medium, however, a polarizedlight component in the specific direction in reflected light from theoptical recording medium which is received at the photosensor is reducedwhile a polarized light component in a direction orthogonal to thespecific direction is produced. In case where a polarization opticalsystem is used, only a polarized light component in the specificdirection is received at the photosensor, so that if the protectivelayer of the optical recording medium has birefringence, the amount ofreceived light at the photosensor is reduced. In case where anon-polarization optical system is used, a polarized light component ina specific direction and a polarized light component in a directionorthogonal to the specific direction are received at the photosensor atthe same ratio. Even if the protective layer of the optical recordingmedium has birefringence, therefore, the amount of light received at thephotosensor does not change. Therefore, the relative amount of receivedlight in case where there is birefringence, with the amount of receivedlight in case of no birefringence present taken as a reference, can beacquired from the ratio of the amounts of received lights in thepolarization optical system and the non-polarization optical system.Because the utilization factor of light from the optical recordingmedium to the photosensor differs between the polarization opticalsystem and the non-polarization optical system, it is preferable thatthe amount of received light in each optical system should be normalizedby the amount of received light with respect to the glass disk whoseprotective layer does not have birefringence.

[0061] While the influence of the in-plane birefringence does not dependon the incident angle, the influence of the perpendicular birefringencedoes, and the light with an incident angle of 0° is not influenced butthe influence gets greater as the incident angle increases. In case of apolarization optical system whose objective lens has a low numericalaperture (NA), the incident angle to the optical recording medium issmall so that the in-plane birefringence decreases the amount ofreceived light while a reduction in the amount of received light causedby the perpendicular birefringence is small. If the numerical aperture(NA) of the objective lens is sufficiently low, a reduction in theamount of received light caused by the perpendicular birefringence canbe neglected. In case of a polarization optical system whose objectivelens has a high numerical aperture (NA), by way of contrast, theincident angle to the optical recording medium is small in the centerportion of the objective lens but is large around the objective lens.This causes both a reduction in the amount of received light originatedfrom the influence of the in-plane birefringence and a reduction in theamount of received light originated from the influence of theperpendicular birefringence.

[0062] As apparent from the above, (A_(PL)/B_(PL))/(A_(NL)/B_(NL)) or L∥represents the relative amount of received light including the influenceof only the in-plane birefringence with the amount of received light incase of no birefringence present as a reference, whereas(A_(PH)/B_(PH))/(A_(NH)/B_(NH)) represents the relative amount ofreceived light including both the influences of the in-planebirefringence and perpendicular birefringence with the amount ofreceived light in case of no birefringence present as a reference.Because the latter expression is the product of the former expressionand the relative amount of received light including the influence ofonly the perpendicular birefringence, the relative amount of receivedlight including the influence of only the perpendicular birefringence isexpressed by[(A_(PH)/B_(PH))/(A_(NH)/B_(NH))]/[(A_(PL)/B_(PL))/(A_(NL)/B_(NL))] orL⊥. Therefore, measuring L∥ and L⊥ is to separately measure the in-planebirefringence and perpendicular birefringence in the birefringencecharacteristic of the protective layer of the optical recording medium.

[0063] With regard to in-plane birefringence, a method of measuring thebirefringence characteristic of the protective layer of an opticalrecoding medium according to another embodiment of the inventionmeasures the birefringence itself. With regard to perpendicularbirefringence, on the other hand, this measuring method irradiatesoutput light from the light source to an optical recording medium viathe objective lens and measures the amount of received light at thephotosensor upon reception of reflected light from the optical recordingmedium via the objective lens. First, the in-plane birefringence ismeasured by the method that has been described earlier referring toFIG. 1. As will be discussed later, the relationship between thein-plane birefringence Δn∥ and the relative amount of received lightincluding the influence of only the in-plane birefringence is acquiredby computation, so that the measured Δn∥ can be converted to L∥. Next,the amounts of received lights A_(PH), B_(PH), A_(NH) and B_(NH) aremeasured using a polarization optical system whose objective lens has ahigh numerical aperture (NA) and a non-polarization optical system whoseobjective lens has a high numerical aperture (NA). The relative amountof received light L⊥ including the influence of only the perpendicularbirefringence is acquired from L∥ and those amounts of received lightsby the following equation.

L⊥=[(A _(PH) /B _(PH))/(A _(NH) /B _(NH))]/L∥

[0064] In the embodiment, it is unnecessary to use a polarizationoptical system whose objective lens has a low numerical aperture (NA)and non-polarization optical system whose objective lens has a lownumerical aperture (NA).

[0065]FIG. 3 is a diagram illustrating the structure of a polarizationoptical system. A polarization beam splitter 3 works to almostcompletely pass a P-polarized light component in input light but almostcompletely reflect an S-polarized light component. The output light froma semiconductor laser 1 is converted by a collimator lens 2 to parallellight which is input as P-polarized light to the polarization beamsplitter 3 and nearly completely passes through it. The light thenpasses a quarter-wave plate 4 to be converted from linearly polarizedlight to circularly polarized light which is focused on a disk 6 by anobjective lens 5. The disk 6 is a target disk for measurement or a glassdisk. In case where the protective layer of the disk 6 does not havebirefringence, reflected light from the disk 6 passes through anobjective lens 5 in the reverse direction and passes through thequarter-wave plate 4 to be converted from circularly polarized light tolinearly polarized light whose polarization direction is orthogonal tothe forward path. Next, the linearly polarized light enters thepolarization beam splitter 3 as S-polarized light and is nearlycompletely reflected, further passes through a cylindrical lens 7 and alens 8 to be received at a photosensor 9. The photosensor 9 is locatedin a middle between two focal lines of the cylindrical lens 7 and lens8.

[0066] In case where the protective layer of the disk 6 hasbirefringence, reflected light from the disk 6 passes through thequarter-wave plate 4 to be converted to elliptically polarized light.While the S-polarized light component is almost completely reflected atthe polarization beam splitter 3 and is received at the photosensor 9,the P-polarized light component nearly completely passes through thepolarization beam splitter 3 and returns to the semiconductor laser 1.That is, in this optical system, of the reflected light from the disk 6,only the S-polarized light component with respect to the polarizationbeam splitter 3 is received at the photosensor 9.

[0067] The numerical aperture (NA) of the objective lens 5 is set to thesame value (e.g., 0.6) as the numerical aperture of the objective lensin an optical information recording/reproducing apparatus which actuallyperforms recording and playback on a target disk for measure in case ofa high numerical aperture (equal to or higher than a predeterminednumerical aperture), but is set to a sufficiently low value (e.g., 0.3)such that a reduction in the amount of received light caused by theinfluence of perpendicular birefringence in the polarization opticalsystem is negligible in case of a low numerical aperture (less than thepredetermined numerical aperture). At this time, instead of using theobjective lens 5 with a low numerical aperture, an objective lens havinga high numerical aperture may be used as the objective lens 5 with anunillustrated aperture restricting element having an aperture smaller indiameter than the effective diameter of. the objective lens 5 insertedbetween the quarter-wave plate 4 and the objective lens 5, therebylowering the effective numerical aperture of the objective lens 5. Thewavelength of the semiconductor laser 1 is set to the same value (e.g.,412 nm) as the wavelength of the light source in the optical informationrecording/reproducing apparatus which actually performs recording andplayback on a target disk for measure.

[0068]FIG. 4 is a diagram illustrating the structure of anon-polarization optical system. Partial polarization beam splitters 10and 12 work to pass about 50% of a P-polarized light component in inputlight and reflect about 50% of the P-polarized light component, andalmost completely reflect an S-polarized light component. The outputlight from a semiconductor laser 1 is converted by the collimator lens 2to parallel light which is input as P-polarized light to the partialpolarization beam splitter 10. About 50% of the P-polarized lightcomponent passes through the beam splitter 10, passes the quarter-waveplate 4 to be converted from linearly polarized light to circularlypolarized light which is focused on the disk 6 by an objective lens 5.The disk 6 is a target disk for measurement or a glass disk. In casewhere the protective layer of the disk 6 does not have birefringence,reflected light from the disk 6 passes through the objective lens 5 inthe reverse direction and passes through the quarter-wave plate 4 to beconverted from circularly polarized light to linearly polarized lightwhose polarization direction is orthogonal to the forward path. Next,the linearly polarized light enters the partial polarization beamsplitter 10 as S-polarized light and is nearly completely reflected,further passes through a half-wave plate 11 so that its polarizationdirection changes by 90°. The light then enters the partial polarizationbeam splitter 12 as P-polarized light about 50% of which is reflectedand further passes through the cylindrical lens 7 and lens 8 to bereceived at the photosensor 9. The photosensor 9 is located in a middlebetween two focal lines of the cylindrical lens 7 and lens 8.

[0069] In case where the protective layer of the disk 6 hasbirefringence, reflected light from the disk 6 passes through thequarter-wave plate 4 to be converted to elliptically polarized light.The S-polarized light component is almost completely reflected at thepartial polarization beam splitter 10 and passes through the half-waveplate 11, changing its polarization direction by 90°. The light thenenters the partial polarization beam splitter 12 as P-polarized lightabout 50% of which is reflected and received at the photosensor 9. Onthe other hand, about 50% of the P-polarized light component isreflected at the partial polarization beam splitter 10 and passesthrough the half-wave plate 11, changing its polarization direction by90°. The light then enters the partial polarization beam splitter 12 asS-polarized light and is almost completely reflected and received at thephotosensor 9. That is, in this optical system, of the reflected lightfrom the disk 6, the S-polarized light component and P-polarized lightcomponent with respect to the partial polarization beam splitter 10 arereceived at the photosensor 9 each by a ratio of 50%.

[0070] The numerical aperture (NA) of the objective lens 5 is set to thesame value (e.g., 0.6) as the numerical aperture of the objective lensin an optical information recording/reproducing apparatus which actuallyperforms recording and playback on a target disk for measure in case ofa high numerical aperture (equal to or higher than a predeterminednumerical aperture), but is set to a sufficiently low value (e.g., 0.3)such that a reduction in the amount of received light caused by theinfluence of perpendicular birefringence in the polarization opticalsystem is negligible in case of a low numerical aperture (less than thepredetermined numerical aperture). At this time, instead of using theobjective lens 5 with a low numerical aperture, an objective lens havinga high numerical aperture may be used as the objective lens 5 with anunillustrated aperture restricting element having an aperture smaller indiameter than the effective diameter of the objective lens 5 insertedbetween the quarter-wave plate 4 and the objective lens 5, therebylowering the effective numerical aperture of the objective lens 5. Thewavelength of the semiconductor laser 1 is set to the same value (e.g.,412 nm) as the wavelength of the light source in the optical informationrecording/reproducing apparatus which actually performs recording andplayback on a target disk for measure.

[0071] In case of a high numerical aperture (equal to or higher than apredetermined numerical aperture), the numerical aperture (NA) of theobjective lens 5 is set to the same value as the numerical aperture ofthe objective lens in the actual optical informationrecording/reproducing apparatus, and the wavelength of the semiconductorlaser 1 is set to the same wavelength as the wavelength of the lightsource in the actual optical information recording/reproducingapparatus, so that the birefringence characteristic of the protectivelayer of the optical recording medium can be measured separately as intoin-plane birefringence and perpendicular birefringence under the sameconditions as the actual optical information recording/reproducingapparatus.

[0072]FIG. 5 is a diagram illustrating the structure of the photosensor9. The reflected light from the disk 6 forms a light spot 14 on lightreceiving sections 13 a to 13 d quadrisected by a dividing line parallelto the radial direction of the disk 6 and a dividing line parallel tothe tangential direction. Provided that outputs from the light receivingsections 13 a to 13 d are given by V13a to V13d, respectively, a focuserror signal is acquired from an arithmetic operation of(V13a+V13d)−(V13b+V13c) by an astigmatism method. A track error signalis acquired from a phase difference between (V13a+V13d) and (V13b+V13c)by a differential phase detection method in case where the disk 6 is ofa read-only type and is acquired from an arithmetic operation of(V13a+V13b)−(V13c+V13d) by a push-pull method in case where the disk 6is of a recordable type or re-recordable type. A sum signal equivalentto the amount of received light at the photosensor 9 is acquired from anarithmetic operation of (V13a+V13b+V13c+V13d). The amount of receivedlight at the photosensor 9 is measured as a focus servo and/or trackservo is applied as needed.

[0073] A description will now be given of the relationship among thein-plane birefringence and perpendicular birefringence in a polarizationoptical system and the amount of received light and the peak intensity.With the X axis and Y axis defined in a cross section perpendicular tothe optical axis, as shown in FIG. 2, the P-polarized light componentand S-polarized light component with respect to the polarization beamsplitter 3 are expressed by using a Jones vector. Provided that theelectric field distribution of the output light from the semiconductorlaser 1 is E₀(x,y) and the Jones matrix of the quarter-wave plate 4 is Qand the Jones matrix of the protective layer of the disk 6 is S, theJones vector of the light which has come out of the polarization beamsplitter 3 and has passed through the quarter-wave plate 4 and theprotective layer of the disk 6 in the forward path to the disk 6 fromthe semiconductor laser 1 is given by the following equation.$\begin{matrix}{\begin{pmatrix}{E_{p0}\left( {x,y} \right)} \\{E_{s0}\left( {x,y} \right)}\end{pmatrix} = {S \cdot Q \cdot \begin{pmatrix}{E_{0}\left( {x,y} \right)} \\0\end{pmatrix}}} & {{Eq}.\quad 1}\end{matrix}$

[0074] The Jones vector of the light which has passed through theprotective layer of the disk 6 and the quarter-wave plate 4 and hasentered the polarization beam splitter 3 in the return path to thephotosensor 9 from the disk 6 is given by the following equation.$\begin{matrix}{\begin{pmatrix}{E_{p}\left( {x,y} \right)} \\{E_{s}\left( {x,y} \right)}\end{pmatrix} = {Q^{*} \cdot S^{*} \cdot \begin{pmatrix}{E_{p0}^{*}\left( {{- x},{- y}} \right)} \\{E_{s0}^{*}\left( {{- x},{- y}} \right)}\end{pmatrix}}} & {{Eq}.\quad 2}\end{matrix}$

[0075] Q and S are given by the following equations. $\begin{matrix}\begin{matrix}{Q = {\frac{1}{\sqrt{2}}\begin{pmatrix}1 & {- i} \\{- i} & 1\end{pmatrix}}} \\{S = \begin{pmatrix}A & B \\B & C\end{pmatrix}} \\{{A = {{\cos \frac{\alpha}{2}} + {i\quad \cos \quad 2\left( {\theta + \phi} \right)\sin \frac{\alpha}{2}}}}\quad} \\{B = {i\quad \sin \quad 2\left( {\theta + \phi} \right)\sin \frac{\alpha}{2}}} \\{C = {{\cos \frac{\alpha}{2}} - {i\quad \cos \quad 2\left( {\theta + \phi} \right)\sin \frac{\alpha}{2}}}} \\{\phi = {\tan^{- 1}\frac{y}{x}}}\end{matrix} & {{Eq}.\quad 3}\end{matrix}$

[0076] Considering an ellipse which is a cross section perpendicular tothe light beam of an index ellipsoid on the protective layer of the disk6, α is the birefringence-originated phase difference between apolarized light component in the direction of the long axis of theellipse and a polarized light component in the direction of the shortaxis and θ is an angle representing the direction of the long axis ofthe ellipse or the direction of the short axis. How to obtain α and θ isdescribed in “Optics”, Vol. 15, No. 5, pp. 414 to 421.

[0077] Let L be the amount of received light at the photosensor 9. Then,L is given by the following equation.

L∝∫∫|E_(s)(x,y)|²dxdy   Eq. 4

[0078] Let P be the peak intensity of the focused spot. Then, P is givenby the following equation.

P∝|∫∫E_(p0)(x,y)dxdy|²+|∫∫E_(s0)(x,y)dxdy|²   Eq. 5

[0079] As the Jones matrix S of the protective layer of the disk 6 is afunction of the in-plane birefringence Δn∥ and the perpendicularbirefringence Δn⊥, L and P are also functions of Δn∥ and Δn⊥.

[0080]FIG. 6 shows an example of computation of the relationship betweenin-plane birefringence and the relative amount of received light. Thehorizontal axis in FIG. 6 indicates the in-plane birefringence Δn∥ andthe vertical axis is the relative amount of received light (L∥) obtainedby normalizing the amount of received light L in case where onlyin-plane birefringence is present by the amount of received light incase of no birefringence present. The calculation conditions indicatedby black circles are the wavelength of the light source being 412 nm andthe numerical aperture (NA) of the objective lens being 0.6, which areequivalent to those of the embodiment of the optical informationrecording/reproducing apparatus according to the invention and thecalculation conditions indicated by white circles are the wavelength ofthe light source being 650 nm, the numerical aperture (NA) of theobjective lens being 0.6, which are equivalent to the DVDspecifications. The thickness of the protective layer of the opticalrecording medium being 0.6 mm. It is apparent that as the in-planebirefringence increases, the relative amount of received light decreasesand the shorter the wavelength of the light source is, the greater thedegree of the reduction in the relative amount of received lightbecomes.

[0081]FIG. 7 shows an example of computation of the relationship betweenin-plane birefringence and the relative peak intensity. The horizontalaxis in FIG. 7 indicates the in-plane birefringence Δn∥ and the verticalaxis is the relative peak intensity obtained by normalizing the peakintensity P in case where only in-plane birefringence is present by thepeak intensity in case of no birefringence present. The calculationconditions indicated by black circles are the wavelength of the lightsource being 412 nm and the numerical aperture (NA) of the objectivelens being 0.6, which are equivalent to those of the embodiment of theoptical information recording/reproducing apparatus according to theinvention and the calculation conditions indicated by white circles arethe wavelength of the light source being 650 nm, the numerical aperture(NA) of the objective lens being 0.6, which are equivalent to the DVDspecifications. The thickness of the protective layer of the opticalrecording medium being 0.6 mm. It is apparent that as the in-planebirefringence increases, the relative peak intensity decreases and theshorter the wavelength of the light source is, the greater the degree ofthe reduction in relative peak intensity becomes.

[0082] The allowance for birefringence described in the specificationsof DVD-ROM, DVD-R and DVD-RW is the allowance for in-plane birefringenceand is expressed as Δn∥≦8.3×10⁻⁵ in terms of Δn∥. This value, whenconverted to the allowance for the relative amount of received lightfrom FIG. 6, becomes L∥≧0.79. According to an optical recording mediumwhose protective layer has a thickness of 0.6 mm and whose recording andplayback are carried out by an optical information recording/reproducingapparatus whose light source has a wavelength of 412 nm and whoseobjective lens has a numerical aperture (NA) of 0.6, the allowance forthe relative amount of received light in case where there is in-planebirefringence should at least be the same as that of DVDs in order toachieve the recording density of 8.2 Gbits/inch² which is equivalent tothe square of the wavelength ratio according to the DVD standard. Thatis, the allowance for the relative amount of received light becomesL∥≧0.79. This value, when converted to the allowance for in-planebirefringence from FIG. 5, becomes Δn∥≦5.3×10⁻⁵ (Δn∥·2d≦64 nm). Theallowance for in-plane birefringence is basically the same for the casewhere the wavelength of the light source is set to 412 nm or shorter andthe numerical aperture (NA) of the objective lens is set to 0.6 orhigher to increase the recording density accordingly to 8.2 Gbits/inch²or greater. In the optical recording medium according to the embodimentof the invention, as the relative amount of received light satisfiesthis allowance and is L∥≧0.79, it is possible to achieve the recordingdensity of 8.2 Gbits/inch² necessary to record and play backhigh-definition moving pictures for 120 minutes.

[0083]FIG. 8 shows an example of computation of the relationship betweenperpendicular birefringence and the relative amount of received light.The horizontal axis in FIG. 8 indicates the perpendicular birefringenceΔn⊥ and the vertical axis is the relative amount of received light (L⊥)obtained by normalizing the amount of received light L in case whereonly perpendicular birefringence is present by the amount of receivedlight in case of no birefringence present. The calculation conditionsare the wavelength of the light source being 412 nm and the numericalaperture (NA) of the objective lens being 0.6, which are equivalent tothose of the embodiment of the optical information recording/reproducingapparatus according to the invention. The thickness of the protectivelayer of the optical recording medium is 0.6 mm. It is apparent that asthe perpendicular birefringence increases, the relative amount ofreceived light becomes smaller.

[0084]FIG. 9 shows an example of computation of the relationship betweenperpendicular birefringence and the relative peak intensity. Thehorizontal axis in FIG. 9 indicates the perpendicular birefringence Δn⊥and the. vertical axis is the relative peak intensity obtained bynormalizing the peak intensity P in case where only perpendicularbirefringence is present by the peak intensity in case of nobirefringence present. The calculation conditions are the wavelength ofthe light source being 412 nm and the numerical aperture (NA) of theobjective lens being 0.6, which are equivalent to those of theembodiment of the optical information recording/reproducing apparatusaccording to the invention. The thickness of the protective layer of theoptical recording medium is 0.6 mm. It is apparent that as theperpendicular birefringence increases, the relative peak intensity getslower.

[0085] According to the DVD specifications, the allowance forperpendicular birefringence is not defined. Because a reduction in peakintensity caused by perpendicular birefringence increases the opticalpower needed at the time of recording, however, the allowance forperpendicular birefringence should be determined. A reduction in peakintensity is also caused by wavefront aberration of a focused spotbesides the birefringence. With regard to the wavefront aberration of afocused spot, an allowance called the Marechal's criterion is defined asdescribed on page 840 in “Optical Technology Handbook” (AsakuraBookstore). This is the allowance for RMS (Root Mean Square) wavefrontaberration when the allowance for the peak intensity of a focused spotis 80% of that in case where there is no wavefront aberration, and isequivalent to about 0.07 λ where λ is the wavelength of the lightsource. With regard to perpendicular birefringence, therefore, it isreasonable to determine the allowance on the premise that the allowancefor the peak intensity of a focused spot is 80% of that in case wherethere is no birefringence as in the case of the wavefront aberration.The allowance for perpendicular birefringence at this time isΔn⊥≦1.3×10⁻³ as apparent from FIG. 9. This value, when converted to theallowance for relative amount of received light, becomes L⊥≧0.57 fromFIG. 8.

[0086] That is, according to an optical recording medium whoseprotective layer has a thickness of 0.6 mm and whose recording andplayback are carried out by an optical information recording/reproducingapparatus whose light source has a wavelength of 412 nm and whoseobjective lens has a numerical aperture (NA) of 0.6, the allowance forthe relative amount of received light in case where there isperpendicular birefringence should be L⊥≧0.57 in order to suppress areduction in peak intensity caused by perpendicular birefringence, whichincreases the optical power needed at the time of recording, and achievethe recording density of 8.2 Gbits/inch². The allowance for the relativeamount of received light is basically the same for the case where thewavelength of the light source is set to 412 nm or shorter and thenumerical aperture (NA) of the objective lens is set to 0.6 or higher toincrease the recording density accordingly to 8.2 Gbits/inch² orgreater. In the optical recording medium according to the embodiment ofthe invention, as the relative amount of received light satisfies thisallowance and is L⊥≧0.57, it is possible to achieve the recordingdensity of 8.2 Gbits/inch² necessary to record and play backhigh-definition moving pictures for 120 minutes.

[0087]FIG. 10 shows another example of computation of the relationshipbetween perpendicular birefringence and the relative amount of receivedlight. The horizontal axis in FIG. 10 indicates the perpendicularbirefringence Δn⊥ and the vertical axis is the relative amount ofreceived light (L⊥) obtained by normalizing the amount of received lightL in case where only perpendicular birefringence is present by theamount of received light in case of no birefringence present. Thecalculation conditions are the wavelength of the light source being 412nm and the numerical aperture (NA) of the objective lens being 0.3. Thethickness of the protective layer of the optical recording medium is 0.6mm. The numerical aperture (NA) of the objective lens 5 in thepolarization optical system and the non-polarization optical system,which are used in the method of measuring the birefringencecharacteristic of the protective layer of the optical recording mediumaccording to the embodiment of the invention, should be set to a valuelow enough that a reduction in the amount of received light caused bythe influence of perpendicular birefringence in the polarization opticalsystem can be neglected. If the numerical aperture (NA) of the objectivelens 5 is set to 0.3, for example, a reduction in the amount of receivedlight when the perpendicular birefringence is 1.3×10⁻³ or smaller is assmall as about 5% and is thus negligible.

[0088]FIG. 11 shows an optical information recording/reproducingapparatus according to one embodiment of the invention. The structure ofthe optical system that guides output light from the semiconductor laser1 to the disk 6 and the structure of the optical system that guidesreflected light from the disk 6 to the photosensor 9 are the same asthose in the polarization optical system shown in FIG. 3. The structureof the photosensor 9 is as illustrated in FIG. 5, and how to carry outarithmetic operations for the focus error signal and track error signaland sum signal are the same as those described earlier referring to FIG.5. The wavelength of the semiconductor laser 1 is 412 nm and thenumerical aperture (NA) of the objective lens 5 is 0.6. The thickness ofthe protective layer of the disk 6 is 0.6 mm and the recording densityis 8.2 Gbits/inch². Further, the relative amounts of received lights onthe disk 6 are L∥≧0.79 and L⊥≧0.57.

[0089] A modulation circuit 16 modulates data to be recorded on the disk6 in accordance with modulation rules. A record signal generatingcircuit 17 generates a record signal to drive the semiconductor laser 1in accordance with a recording strategy. A semiconductor-laser drivecircuit 18 drives the semiconductor laser 1 by supplying the currentcorresponding to the record signal, generated by the record signalgenerating circuit 17, to the semiconductor laser 1 based on the recordsignal. Accordingly, data is recorded on the disk 6. An amplifiercircuit 19 amplifies outputs from the individual light receivingsections of the photosensor 9. A playback-signal processing circuit 20generates, equalizes and digitizes a playback signal (sum signal) basedon the signals amplified by the amplifier circuit 19. A demodulationcircuit 21 demodulates a signal, digitized by the playback-signalprocessing circuit 20, in accordance with demodulation rules.Accordingly, data is played back from the disk 6.

[0090] An error-signal generating circuit 22 generates a focus errorsignal and a track error signal based on the signals amplified by theamplifier circuit 19. An objective-lens drive circuit 23 drives theobjective lens 5 by supplying the current corresponding to errorsignals, generated by the error-signal generating circuit 22, to anunillustrated actuator, based on the error signals. Further, the opticalsystem excluding the disk 6 is driven in the radial direction of thedisk 6 by an unillustrated positioner and the disk 6 is turned by anunillustrated spindle. Accordingly, focus, track, positioner and spindleservos are carried out.

[0091] The circuitry from the modulation circuit 16 to thesemiconductor-laser drive circuit 18, which is associated with datarecording, the circuitry from the amplifier circuit 19 to thedemodulation circuit 21, which is associated with data playback, and thecircuitry from the amplifier circuit 19 to the objective-lens drivecircuit 23, which is associated with servos, are controlled by acontroller 15.

[0092] According to the optical information recording/reproducingapparatus embodying the invention, the relative amount of received lightsatisfies the allowance and recording and playback are performed on anoptical recording medium with L∥≧0.79 and L⊥≧0.57, thus making itpossible to suppress a reduction in the amount of light received at thephotosensor and a reduction in the peak intensity of a focused spot andachieve the recording density of 8.2 Gbits/inch² or higher which isrequired to record and play back high-definition moving pictures for 120minutes.

What is claimed is:
 1. A birefringence characteristic measuring methodcomprising the steps of: irradiating light onto a target medium formeasurement via an objective lens having a numerical aperture equal toor higher than a predetermined numerical aperture, and measuring anamount of light of a polarized light component in a specific direction,which is included in reflected light reflected at a reflection surfaceof said target medium to thereby acquire a first amount of light A_(PH);irradiating light onto said target medium via said objective lens havingsaid numerical aperture equal to or higher than said predeterminednumerical aperture, and measuring an amount of light of a polarizedlight component in said specific direction, which is included inreflected light reflected at said reflection surface of said targetmedium, and an amount of light of a polarized light component in adirection orthogonal to said specific direction to thereby acquire asecond amount of light A_(NH); and acquiring a perpendicularbirefringence characteristic of said target medium based on a ratioA_(PH)/A_(NH) of said first amount of light to said second amount oflight and an in-plane birefringence characteristic of said targetmedium.
 2. The birefringence characteristic measuring method accordingto claim 1, further comprising the steps of: irradiating light onto saidtarget medium via an objective lens having a numerical aperture lowerthan said predetermined numerical aperture, and measuring an amount oflight of a polarized light component in a specific direction, which isincluded in reflected light reflected at said reflection surface of saidtarget medium to thereby acquire a third amount of light A_(PL);irradiating light onto said target medium via said objective lens havingsaid numerical aperture lower than said predetermined numericalaperture, and measuring an amount of light of a polarized lightcomponent in said specific direction, which is included in reflectedlight reflected at said reflection surface of said target medium, and anamount of light of a polarized light component in a direction orthogonalto said specific direction to thereby acquire a fourth amount of lightA_(NL); and acquiring said in-plane birefringence characteristic basedon a ratio A_(PL)/A_(NL) of said third amount of light to said fourthamount of light.
 3. The birefringence characteristic measuring methodaccording to claim 1, wherein said measuring of said first amount oflight is a step of irradiating said reflected light onto a photosensorvia a polarization optical system which outputs a polarized lightcomponent in said specific direction in input light and; and saidmeasuring of said second amount of light is a step of irradiating saidreflected light onto a photosensor via a non-polarization optical systemwhich outputs a polarized light component in said specific direction ininput light and a polarized light component in a direction orthogonal tosaid specific direction at an approximately same ratio.
 4. Thebirefringence characteristic measuring method according to claim 2,wherein said measuring of said first amount of light or said thirdamount of light is a step of irradiating said reflected light onto aphotosensor via a polarization optical system which outputs a polarizedlight component in said specific direction in input light and; and saidmeasuring of said second amount of light or said fourth amount of lightis a step of irradiating said reflected light onto a photosensor via anon-polarization optical system which outputs a polarized lightcomponent in said specific direction in input light and a polarizedlight component in a direction orthogonal to said specific direction atan approximately same ratio.
 5. The birefringence characteristicmeasuring method according to claim 1, further comprising the step ofmeasuring first and second amounts of light for normalization B_(PH) andB_(NH) respectively corresponding to said first and second amounts oflight using a reference medium which does not substantially havebirefringence in place of said target medium, converting Δn∥ which is avalue of in-plane birefringence of said target medium to a relativeamount of received light L∥ including influence of only said in-planebirefringence and acquiring said perpendicular birefringencecharacteristic from L⊥=[(A_(PH)/B_(PH))/(A_(NH)/B_(NH))]/L∥.
 6. Thebirefringence characteristic measuring method according to claim 3,further comprising the step of measuring first and second amounts oflight for normalization B_(PH) and B_(NH) respectively corresponding tosaid first and second amounts of light using a reference medium whichdoes not substantially have birefringence in place of said targetmedium, converting Δn∥ which is a value of in-plane birefringence ofsaid target medium to a relative amount of received light L∥ includinginfluence of only said in-plane birefringence and acquiring saidperpendicular birefringence characteristic fromL⊥=[(A_(PH)/B_(PH))/(A_(NH)/B_(NH))]/L∥.
 7. The birefringencecharacteristic measuring method according to claim 2, further comprisingthe step of measuring first to fourth amounts of light for normalizationB_(PH), B_(NH), B_(PL) and B_(NL) respectively corresponding to saidfirst to fourth amounts of light using a reference medium which does notsubstantially have birefringence in place of said target medium,acquiring said in-plane birefringence characteristic fromL∥=(A_(PL)/B_(PL))/(A_(NL)/B_(NL)), and acquiring said perpendicularbirefringence characteristic fromL⊥=[(A_(PH)/B_(PH))/(A_(NH)/B_(NH))]/L∥.
 8. The birefringencecharacteristic measuring method according to claim 4, further comprisingthe step of measuring first to fourth amounts of light for normalizationB_(PH), B_(NH), B_(PL) and B_(NL) respectively corresponding to saidfirst to fourth amounts of light using a reference medium which does notsubstantially have birefringence in place of said target medium,acquiring said in-plane birefringence characteristic fromL∥=(A_(PL)/B_(PL))/(A_(NL)/B_(NL)) , and acquiring said perpendicularbirefringence characteristic fromL⊥=[(A_(PH)/B_(PH))/(A_(NH)/B_(NH))]/L∥.
 9. A birefringencecharacteristic measuring method of measuring a birefringencecharacteristic of a protective layer of an optical recording mediumusing a birefringence characteristic measuring apparatus having a lightsource, a photosensor, and an optical system which includes a beamsplitter, a quarter-wave plate for passing light output from said lightsource and passed through said beam splitter and an objective lens forirradiating said light, transmitted through said quarter-wave plate,onto a target optical recording medium for measurement and which guidesreflected light reflected from said target optical recording medium tosaid photosensor via said objective lens, said quarter-wave plate andsaid beam splitter, said optical system being switched among a firstoptical system which guides only a polarized light component in aspecific direction in said reflected light reflected from said targetoptical recording medium using an objective lens having a numericalaperture equal to or higher than a predetermined numerical aperture, asecond optical system which guides only a polarized light component insaid specific direction in said reflected light reflected from saidtarget optical recording medium using an objective lens having anumerical aperture lower than said predetermined numerical aperture, athird optical system which guides a polarized light component in saidspecific direction and a polarized light component in a directionorthogonal to said specific direction, both included in said reflectedlight reflected from said target optical recording medium, at anapproximately same ratio, using an objective lens having a numericalaperture equal to or higher than said predetermined numerical aperture,and a fourth optical system which guides a polarized light component insaid specific direction and a polarized light component in a directionorthogonal to said specific direction, both included in said reflectedlight reflected from said target optical recording medium, at anapproximately same ratio, using an objective lens having a numericalaperture lower than said predetermined numerical aperture.
 10. Thebirefringence characteristic measuring method according to claim 9,wherein amounts of received light measured for said target opticalrecording medium using said first to fourth optical systems arenormalized respectively based on amounts of received light measured fora reference optical recording medium having a protective layer whichdoes not substantially have birefringence using said first to fourthoptical systems.
 11. The birefringence characteristic measuring methodaccording to claim 10, wherein given that said amounts of received lightmeasured for said target optical recording medium by said first tofourth optical systems are respectively A_(PH), A_(PL), A_(NH) andA_(NL) and said amounts of received light measured for said referenceoptical recording medium by said first to fourth optical systems arerespectively B_(PH), B_(PL), B_(NH) and B_(NL), an in-planebirefringence characteristic and a perpendicular birefringencecharacteristic of said target optical recording medium are acquiredrespectively from L∥=(A _(PL) /B _(PL))/(A _(NL) /B _(NL)), and L⊥=[(A_(PH) /B _(PH))/(A _(NH) /B _(NH))]/[(A _(PL) /B _(PL))/(A _(NL) /B_(NL))].
 12. An optical recording medium whose recording or playback iscarried out at a recording density of 8.2 Gbits/inch² or higher using anoptical information recording/reproducing apparatus having a lightsource with a wavelength of 412 nm or less and an objective lens with anumerical aperture of 0.6 or higher, and which has a protective layerwith a thickness of about 0.6 mm through which light is transmitted andwhose L∥ measured by said birefringence characteristic measuring methodas recited in claim 7 is L∥≧0.79.
 13. An optical recording medium whoserecording or playback is carried out at a recording density of 8.2Gbits/inch² or higher using an optical information recording/reproducingapparatus having a light source with a wavelength of 412 nm or less andan objective lens with a numerical aperture of 0.6 or higher, and whichhas a protective layer with a thickness of about 0.6 mm through whichlight is transmitted and whose L⊥ measured by said birefringencecharacteristic measuring method as recited in claim 7 is L⊥≧0.57.
 14. Anoptical recording medium whose recording dr playback is carried out at arecording density of 8.2 Gbits/inch² or higher using an opticalinformation recording/reproducing apparatus having a light source with awavelength of 412 nm or less and an objective lens with a numericalaperture of 0.6 or higher and which has a protective layer with athickness of about 0.6 mm through which light is transmitted and inwhich Δn∥·2d≦64 nm where Δn∥ is a value of in-plane birefringence ofsaid protective layer and d is a thickness of said protective layer. 15.An optical recording medium whose recording or playback is carried outat a recording density of 8.2 Gbits/inch² or higher using an opticalinformation recording/reproducing apparatus having a light source with awavelength of 412 nm or less and an objective lens with a numericalaperture of 0.6 or higher and which has a protective layer with athickness of about 0.6 mm through which light is transmitted and whoseL⊥ measured by said birefringence characteristic measuring method asrecited in claim 5 is L⊥≧0.57.
 16. An optical informationrecording/reproducing apparatus that has a light source with awavelength of 412 nm or less and an objective lens with a numericalaperture of 0.6 or higher and records or plays back, at a recordingdensity of 8.2 Gbits/inch² or higher, an optical recording medium whichhas a protective layer with a thickness of about 0.6 mm through whichlight is transmitted and whose L∥ measured by said birefringencecharacteristic measuring method as recited in claim 7 is L∥≧0.79.
 17. Anoptical information recording/reproducing apparatus that has a lightsource with a wavelength of 412 nm or less and an objective lens with anumerical aperture of 0.6 or higher and records or plays back, at arecording density of 8.2 Gbits/inch² or higher, an optical recordingmedium which has a protective layer with a thickness of about 0.6 mmthrough which light is transmitted and whose L⊥ measured by saidbirefringence characteristic measuring method as recited in claim 7 isL⊥≧0.57.
 18. An optical information recording/reproducing apparatus thathas a light source with a wavelength of 412 nm or less and an objectivelens with a numerical aperture of 0.6 or higher and records or playsback, at a recording density of 8.2 Gbits/inch² or higher, an opticalrecording medium which has a protective layer with a thickness of about0.6 mm through which light is transmitted and in which Δn∥·2d≦64 nmwhere Δn∥ is a value of in-plane birefringence of said protective layerand d is said thickness of said protective layer.
 19. An opticalinformation recording/reproducing apparatus that has a light source witha wavelength of 412 nm or less and an objective lens with a numericalaperture of 0.6 or higher and records or plays back, at a recordingdensity of 8.2 Gbits/inch² or higher, an optical recording medium whichhas a protective layer with a thickness of about 0.6 mm through whichlight is transmitted and whose L⊥ measured by said birefringencecharacteristic measuring method as recited in claim 5 is L⊥≧0.57.